In this particular case the fifteen year old kid is well spoken... combine that with the implied business ownership AND the fact that the transaction was initiated on an auction site with an 18+ clause in their TOS and it's evident that the kid was misrepresenting his age.
With regards to being forced to extend or honor any sort of guarantee... Well, alive and healthy are kind of givens, as the animal was advertised as being such, sold as being such and sending a dead animal would be fraud... the seller is responsible for the health and well being of the animal until the buyer accepts ownership and care- the transit duration is a seller's responsibility. This can sometimes be different in cases where large numbers of animals are sent in a wholesale situation or when the lack of a guarantee is specified clearly in advance of the purchase for species that are more likely to experience problems when shipped...
Above and beyond that there's no legal requirement for any guarantee to be offered. The moment the package was recieved and there was confirmation of live, healthy arrival the seller's responsibility ended. Many businesses and breeders CHOOSE to offer an extended guarantee period but this is entirely of their own devising. Offering a guarantee makes a customer feel more comfortable with their purchase, since the seller is displaying confidence in the quality of their animals. In the above thread no guarantee was discussed however and as such the buyer can't create one and decide it applies after the fact.
Legally since the buyer had the animal shipped to an adult AND misrepresented themselves as being over eighteen there's really no argument in the "Minors can't enter into binding contracts" aspect, since the seller was given (false) evidence that appeared credible and was acting in good faith based on the information provided. The child's mother who really ignited the whole situation didn't have a complaint until after the snake had died with regards to the purchase and the period of time involved is certainly long enough for improper husbandry to kill off a neonate animal. Since the body was frozen, preventing a reliable necropsy that might have proven long term health problems they really have no choice but to accept the loss and eat the money. Add in the fact that they have conducted themselves quite poorly on a public forum, whereas the seller conducted himself in a reasonable and rational manner and it looks very bad for the buyer.
"Genes, Like Leibnitz's monads, have no windows; the higher properties of life are emergent... And once assembled, organisms have no windows." - Edward Wilson, Sociobiology